When your mouth opens your on salivation glands secrete saliva which has enzymes that are used to break down your food for digestion. When your mouth opens these glands don't know if you are eating or not, they just do their job. Likely when you fall asleep your face muscles relax, and your mouth comes open producing the saliva which falls out of your mouth when you sleep. A large amount of saliva might be a sign that you talk in your sleep, since the movement of the jaw produces it at a larger rate.

Hey, that's simple - it's invisible ink to draw the world map on your pillow to strategize of conquering the world or even chart where your next travelling destinations while you're dreaming in your sleep. Through time, you'll notice the world map materialize on your pillow when you're changing your pillow case, hahaha